How Is Steel Made? Iron, Carbon and the Furnace

Iron in the ground is rust, oxides of iron in ores such as haematite and magnetite, and getting the metal out means removing the oxygen. The blast furnace does that with carbon. Ore, coke, which is coal baked to nearly pure carbon, and limestone are fed in at the top of a tower thirty metres high, and hot air is blasted in at the bottom. The coke burns to carbon monoxide, the carbon monoxide strips the oxygen from the ore as the charge descends, and molten iron collects at the bottom with a layer of slag, the limestone having gathered up the ore's rocky impurities, floating on top. The furnace runs without stopping for ten or fifteen years. What comes out is pig iron, about four percent carbon, with silicon, manganese, phosphorus and sulphur, and it is hard, brittle and useless for anything but casting.

Steelmaking is the removal of most of that carbon and the impurities, and the modern method is to blow oxygen through the molten iron. In the basic oxygen furnace, a pear-shaped vessel holding up to 400 tonnes, a water-cooled lance lowered into the melt blows pure oxygen at supersonic speed for about twenty minutes; the carbon burns off as carbon monoxide, the silicon and manganese oxidise into the slag, and the heat of the reactions keeps the metal molten with no fuel at all, so that scrap steel is added to cool it. The process, developed in Austria in 1952, does in twenty minutes what earlier furnaces took hours over, and it makes about seventy percent of the world's steel. The other thirty percent comes from electric arc furnaces, which melt scrap steel with an electric arc and are the main route for recycling; steel is the most recycled material on Earth, and a car's body is likely to have been a car before.

Iron atoms pack in a crystal lattice, and the layers of the lattice slide past each other easily, which is why pure iron is soft. Carbon atoms, much smaller, sit in the gaps between iron atoms and jam the layers, so a little carbon makes the metal harder and stronger; too much and it forms brittle iron carbide throughout and the metal cracks instead of bending. Heat treatment adjusts the result. Steel heated red and quenched in water traps the carbon in a distorted lattice called martensite, glass-hard and brittle, and reheating it gently, tempering, trades some hardness for toughness, which is how a sword or a chisel gets a hard edge on a body that will not shatter. Alloying elements add the rest:

• •Chromium, above about 11 percent, forms a self-healing oxide skin and makes stainless steel
• •Nickel keeps stainless steel tough at low temperatures and is why cutlery is 18/8, eighteen percent chromium and eight nickel
• •Manganese, in nearly all steel, ties up sulphur and adds strength; at 12 percent it makes the steel of railway points and rock crushers, which hardens where it is hit
• •Tungsten, molybdenum and vanadium make tool steel that stays hard red-hot
• •Silicon in transformer steel, boron in armour, and a few hundredths of a percent of niobium in the high-strength steel of pipelines and car bodies

Steel was made in small quantities for two thousand years, in Damascus, in Japan and in the crucibles of Sheffield, by craftsmen who could not have said what carbon was. The industrial age began in 1856 when Henry Bessemer blew air through molten pig iron in a converter and found the carbon burned out in minutes, turning a material that had cost as much as silver into one that could be made by the hundred tonnes; the open-hearth furnace of the 1860s refined the method, and the price of steel fell by four fifths within thirty years. Rails, ships, the Eiffel Tower's rival in Chicago, the first skyscrapers and the armoured warships of the naval race all followed, and the oxygen furnace and continuous casting after 1950 finished the job. China now makes about half the world's steel.

Every tonne of steel from a blast furnace releases about 1.8 tonnes of carbon dioxide, most of it from the coke that reduces the ore, and the process has no alternative reducing agent in commercial use. Two are coming. Hydrogen can strip the oxygen from iron ore as carbon monoxide does, producing water instead of carbon dioxide, and the first hydrogen-reduced steel was delivered in Sweden in 2021, with commercial plants under construction. And electrolysis, which extracts aluminium from its ore with electricity, is being adapted for iron. Both need very large amounts of clean electricity, which is the constraint; until it is lifted, the world's most useful material remains one of its largest sources of the gas that is warming it.

Molten steel is only half made, since it still has to become something solid of a useful shape. Before the 1950s it was poured into individual moulds to cool as ingots, which were reheated and worked later. Today almost all of it is cast continuously: the liquid flows into a bottomless, water-cooled copper mould, a solid skin forms against the cold walls within seconds, and the glowing strand is drawn steadily out of the bottom while its core is still liquid. Water sprays finish the cooling, and torches cut the strand into thick slabs for plate or square billets for bars and wire.

Those pieces are then rolled. In hot rolling, steel at around a thousand degrees is squeezed between pairs of heavy rollers again and again, each pass making it thinner and longer, and the working breaks up the coarse crystals that formed during casting into fine grains, which makes the metal tougher. Sheet for car bodies, food cans and washing machines is often then cold rolled at room temperature, which gives a smoother, more precise surface and hardens it further. Much of it is finally dipped in molten zinc, or galvanised, so that the zinc corrodes first and the steel beneath is protected.

Bessemer's process almost failed at the start. Ironmasters who bought licences found that their steel came out brittle and useless, even though Bessemer's own trials had worked. The difference was phosphorus. Bessemer had happened to test pig iron made from ore almost free of it, but most British and European ores contained enough to make the finished steel crack when cold, and blowing air through the melt did not remove it. For about twenty years, Bessemer steel could be made only from a limited range of costly phosphorus-poor ores, such as the haematite of Cumbria and northern Spain.

The fix came in the late 1870s from Sidney Gilchrist Thomas, a clerk at a London police court who studied chemistry in the evenings, working with his cousin Percy Gilchrist. They lined the converter with a basic material, burnt dolomite or limestone, which reacts with the acidic phosphorus oxides and draws them into the slag. The vast phosphoric ore fields of Lorraine and northern Sweden suddenly became valuable, which helped German steel output overtake Britain's before 1900. The phosphorus-rich slag was ground up and sold as fertiliser, and the word basic in the name of today's basic oxygen furnace refers to the same kind of lining.

Stainless steel was developed in Sheffield. In 1913 the metallurgist Harry Brearley, looking for a steel for gun barrels that would resist wear, experimented with adding chromium. He noticed that some of his samples did not rust or etch in the usual way when exposed to acids that marked ordinary steel.

The secret is a thin, invisible layer of chromium oxide that forms on the surface when the steel contains at least about eleven per cent chromium, and that layer heals itself if scratched. Brearley saw the potential for cutlery, which until then had rusted and stained easily. Similar discoveries were made around the same time in Germany and elsewhere, and stainless steel went on to transform kitchens, hospitals and buildings.

Steel is iron with a small, controlled amount of carbon, made by first reducing iron ore with coke in a blast furnace to brittle pig iron and then burning most of the carbon out with a blast of oxygen, or by melting scrap in an electric arc furnace. The carbon atoms lodged between the iron atoms give the metal its strength, heat treatment and alloying tune it for every use, and the coke that does the reducing is why steelmaking produces eight percent of the world's carbon dioxide, a problem hydrogen is beginning to address.